Gay Lussac’s Law of Combining Volumes states that when gases react, they do so in volumes which bear a simple ratio to one another, and to the volume of the product(s) formed if gaseous, provided the temperature and pressure remain constant.
C2_22H4(g)_{4(g)}4(g) + 3O2(g)_{2(g)}2(g) → 2CO2(g)_{2(g)}2(g) + 2H2_22O(g)_{(g)}(g)
1 mole : 3 moles
Total volume required: 10 cm3^33 50 cm3^33
Reacted Volume: 10 cm3^33 30 cm3^33
Residual volume: 0 (50 - 30) = 20 cm3^33